The invention is particularly related to a method and apparatus for photoelectrically determining the position of at least one focus plane of an image within an optical instrument. The object is reproduced on at least one spatial frequency filter of an optical image correlator and measurement or display of the light fluxes leaving the spatial frequency filter takes place.
The state of the prior art may be ascertained by reference to U.S. Pat. No. 3,781,110 of Leitz et al which issued Dec. 25, 1973, and the references incorporated therein. U.S. Pat. No. 3,781,110 is incorporated herein.
Such methods for photoelectrically determining the focus plane are known. The bundle of light rays from the objective passes through a modulating frequency filter and thereupon is incident upon a photoelectric receiver and the AC output signal thereof is analyzed regarding its amplitude. Furthermore, systems have been proposed, wherein several exit pupils partly overlap one another, or where two exit pupils separately direct light on an associated photoelectric receiver for each. In the first mentioned embodiment, use is made of a single pupil sector when the optical-electronic (optronic) distance measurement is undertaken. In the latter embodiment, use is made summarily of the sector cut-outs on one side, as well as of the symmetrical one on the other of the split-up pupils. These devices do not sufficiently consider the presence of beam centers of gravity between the right and left pupil halves, which though symmetric, nevertheless are different in perspective with respect to the object, that is, there is failure to make full use of ray parallax in the pupils.